The present invention relates to communication devices and, more particularly, to Universal Serial Bus (USB) communication device and methods of operating the same.
A variety of different types of devices communicate using the Universal Serial Bus (USB) standard. For USB bus based communications, one device, such as a personal computer, is generally the host device and supports a USB host controller. Attached devices operate as slave devices in communication with the host controller device. Various modes are known related to power control and the like apart from an operational mode. One such alternative mode is a suspend mode, where the host controller may request that the slave enter a suspend mode with limited current draw on the power line associated with the USB bus. Thus, various conditions arise where the host and slave communicate and the slave is expected to respond to a signal from the host controller.
Some slave devices include various additional functional circuitry unrelated to the USB communication circuitry. An example of a device that may use a USB connection to a host controller is a wireless communication device that is configured to operate as a wireless modem for the host controller device. A common cellular system design implementation for use in such a wireless modem is to generate the reference frequency for the radio in the RF section of the cellular device. The radio reference frequency is then commonly shared with the baseband circuit of the cellular device in order to generate the high frequency master clock for the microprocessor. During the intervals between the paging frames from the cellular network, all unnecessary circuits, including the RF section and much of the baseband circuit of the cellular device may be turned off in order to reduce power consumption. If the cellular device is to communicate with an application by means of a USB interface, the communication link may be disrupted and disabled during these deep sleep states due, for example, to the loss of the high frequency clocks from the cellular devices RF section.
One approach to avoid this condition may be to keep the RF powered on at all times to ensure that the clocks are available to support the USB interface, however, this would generally result in a significant power consumption penalty. Another approach may be to create a hardware (HW) handshake mechanism between the cellular device and the host controller application so that each can wake the other if a message is to be sent. However, this solution may not be compliant with the USB standard.